Exporting to TS3 for testing

I use Paint.net for this since the image needs to be 24 bit png. I found this tutorial for doing it in paintnet, should at least give you an idea of whats involved. http://ts3cc.canadian-forum.com/t297-tutorial-thumbnail-24-bit-png-image-creation-with-paint-net
Or you can find the default icon in Program Files(x86)/Electronic Arts/The Sims 3 Create a World Tool/defaultWorldThumb